Our overall objective is to determine the mechanisms which regulate protein synthesis during cellular differentiation. We have already shown that the synthesis of myosin heavy-chain subunits is highly regulated in quail myoblasts during their differentiation in cell culture. We now propose to examine other muscle proteins to determine whether their syntheses are controlled coordinately with or independently of the synthesis of myosin heavy chains. Specifically, the rates of synthesis of actin, tropomyosin, myosin light chains, and myoglobin will be studied in embryonic quail Coturnix coturnix breast muscle myoblasts during their differentiation in cell culture. Rates of synthesis will be quantitated by measurement of the incorporation of radioactive amino acids into these proteins after immunoprecipitation with protein-specific antisera and subsequent acrylamide gel electrophoresis. The synthesis, accumulation and stability of the myosin heavy chain and tropomyosin messenger RNAs (mRNAs) will also be studied. These experiments will examine whether the synthesis of these proteins is controlled by mechanisms which regulate the transcription of the mRNAs for these proteins or, alternatively, by post-transcriptional mechanisms which regulate either the stabilities or the translation of these mRNAs. These mRNAs will be isolated and purified by immunoprecipitation of myosin heavy-chain and tropomyosin polysomes and by fractionation of polysomal RNA by molecular size, poly A content, and affinity for fractionated initiation factors. Purified mRNAs will be identified by translation in wheat germ and reticulocyte lysates and by analyses of nucleotide sequence complexity and oligonucleotide sequence. The synthesis, accumulation, and stability of these mRNAs during myoblast differentiation will be measured by pulse-chase analyses of the relative incorporation of radioactive nucleosides into these mRNAs and ribosomal RNA, by the ability of RNA extracts to stimulate specific protein synthesis in wheat germ and reticulocyte lysates, and by hybridization of RNA with DNA probes synthesized from purified mRNAs.